Recovery and purification systems for acrylonitrile are known, see for example U.S. Pat. Nos. 3,936,360; 3,433,822; and 3,399,120.
Typically, propylene, ammonia and air are reacted in a vapor phase with an ammoxidation catalyst. The vaporous reactor effluent is then passed to a quench system wherein the reactor effluent is directly contacted with an aqueous quenching liquid, usually water. This quenching removes unreacted ammonia and heavy polymers. The quenched gases then proceed to an absorption column.
In the absorber, the gases are directly contacted with an absorbing liquid, again usually water. The water, acrylonitrile, acetonitrile, HCN and associated impurities leave the bottom of the absorber in an aqueous solution. Inert gases are removed from the top of the absorber.
The aqueous solution then proceeds to a series of distillation columns to separate the respective components. The first of these is known as a recovery column. This column removes acetonitrile from the aqueous solution through extractive distillation. U.S. Pat. No. 3,399,120 to Lovett shows such a recovery column and its associated stripper. Improvements have been made in the prior art to the process of Lovett, such that acetonitrile is now removed as a sidestream from the recovery column.
U.S. Pat. No. 3,936,360 describes the common recovery scheme for further refining the overhead stream from the recovery column containing acrylonitrile, HCN, water and minor impurities. This stream is first passed to an HCN column with HCN being removed overhead; then to a drying column with water being removed overhead; and finally to a product column wherein product quality acrylonitrile is recovered. Improvements have also been made in this aspect of the process by combining the HCN and drying columns into one distillation column.
Associated with each of these columns is the requirement of heat addition to perform the necessary distillation. This heat addition is typically in the form of steam. The present invention greatly reduces the external steam requirements necessary to recover acrylonitrile, thereby substantially reducing the operating costs.